This invention is in the general area of orthopedic prostheses, particularly tibial prostheses.
The tibia is situated at the front and inner side of the leg and, except for the femur, is the longest and largest bone in the skeleton. It is prismoid in form, expanded above, where it enters into the knee joint. The head of the tibia is large and expanded on each side into two eminences, the tuberosities. These present two smooth concave surfaces which articulate with the condyles of the femur. The medial condyle is more prominent anteriorly and broader both in the anterior-posterior and transverse diameters than the lateral condyle. Accordingly, the lateral articular surface of the tibia is longer, deeper and narrower than the medial surface of the tibia so as to articulate with the lateral condyle. The medial surface is broader and more circular, concave from side to side, to articulate with the medial condyle. The anterior surfaces of the tuberosities are continuous with one another, forming a single large surface which is somewhat flattened. Posteriorly the tuberosities are separated from each other by a shallow depression for attachment of ligaments. The inner tuberosity presents posteriorly a deep transverse groove for the insertion of a tendon.
In the past, manufacturers of tibial prostheses have ignored the anatomical differences between the medial and lateral condyle compartments. Most prostheses are constructed of a totally symmetric baseplate component for support and either a left or right knee-specific tibial insert or a surgeon-assembled neutral insert. Although the symmetrical components are interchangeable from the right to the left tibia, they invariably either overhang the lateral tibial bone or are undersized on the medial tibial bone surface.
The symmetrical prostheses generally feature center stems, either porous coated or smooth, or porous coated pegs. Further studies such as "An Evaluation of the Load Bearing Capability of the Cancellous Proximal Tibia with Special Interest in the Design of Knee Implants" by J. A. Johnson et al., 29th Annual ORS, Anaheim, CA, Mar. 8-10, page 403 (1983), show that the strongest portions of the tibia head are located at the lateral and medial portions of the tibia, with the weakest bone located in the central portion. The capacity of the bone to sustain load is important to the long-term survival of the reconstructed knee, and failure of the cancellous bone leads to subsidence of the implanted prosthesis. It would be difficult, if not impossible, to design a totally symmetrically tibial prosthesis with multiple pegs properly positioned since the pegs would either be too close to the outer wall of the bone or located too much towards the inner, soft portion of the bone, depending on whether the implant was on the right or left knee.
The prior prostheses have been designed to be totally symmetrical to minimize inventory costs. Unfortunately, the result is to compromise the fit of the prosthesis. Prior art prostheses have also usually been factory assembled. This is disadvantageous since either a large inventory of prostheses of various dimensions must be maintained or the surgeon must again settle for a less than optimal fit. Another disadvantage is that it is difficult to use bone screws to secure the baseplate of the factory assembled prosthesis.
It is therefore an object of the present invention to provide a tibial prosthesis which closely approximates the natural tibial anatomy.
It is a further object of the present invention to provide a tibial prosthesis with an anatomic fit contributing to the normal range of motion.
It is a still further object of the present invention to provide a tibial prosthesis wherein the long-term stability of the implant, both with respect to adhesion and subsidence, is optimized.
It is another object of the present invention to provide a tibial prosthesis which is extremely biocompatible and strong.
It is still another object of the present invention to provide a surgeon-assembled tibial prosthesis formed from modular components which can be used to provide the closest fit possible to the natural anatomic structure with the minimum inventory expense.